1. Field of the Invention
The present invention relates to a novel silica gel which is excellent in e.g. heat resistance and hydrothermal resistance.
2. Discussion of Background
Silica gel has been widely used as a drying agent and in recent years, it has been used for various purposes as e.g. a catalyst support, a separating medium or an adsorbent. Accordingly, requirements for performances of the silica gel have been diversified according to the purposes. The performances of the silica gel are greatly influenced by physical properties of the silica gel such as the surface area, the pore diameter, the pore volume, the pore size distribution, and these physical properties are greatly influenced by conditions for producing the silica gel.
As a method for producing the silica gel, the most common method is to hydrolyze an alkali silicate such as sodium silicate with a mineral acid, and gelate the resulting silica hydrosol, followed by drying. Many proposals have been made with respect to the details of the production method so as to improve the performances of the silica gel.
For example, JP-A-62-113713 proposes a method for producing a silica gel having a narrow pore distribution, produced by gelling silica hydrosol formed by reaction of alkali silicate aqueous solution and mineral acid solution, treating a silica hydrogel with acid solution at a pH under 2.5, washing with water, adjusting the pH to 4-9 in a buffer solution and treating hydrothermally. In Examples of the above gazette, a silica gel having an average pore diameter of from 6.7 to 8.5 nm and a pore volume of from about 0.8 to about 0.9 ml/g can be obtained by the above method.
Further, JP-A-9-30809 proposes a method wherein a silica hydrogel is dried by batch flow drying and then a hydrothermal treatment is carried out. Changes in performances of the silica gel obtained by this method are also confirmed, and a silica gel having a sharper pore distribution can be obtained. However, the pore volume, the specific surface area and the average pore diameter can not adequately be changed, and this method is inadequate as a method to obtain a silica gel having desired physical properties.
On the other hand, in the silica gel obtained by using an alkali silicate as a material as explained above, usually a considerable amount of impurities such as sodium, calcium, magnesium, titanium, aluminum and zirconium derived from the material is contained. The metal impurities in the silica gel can have a significant influence over the performances of the silica gel even if the total content is so small as at a level of several hundreds ppm. For example, metal impurities can (1) accelerate crystallization of the silica gel at a high temperature, 2) accelerate a hydrothermal reaction of the silica gel in the presence of water to cause an increase in the pore diameter and the pore volume, a decrease in the specific surface area and a broadening in the pore distribution, and 3) decrease the sintering temperature, and accordingly when a silica gel containing them is heated, cause a decrease in the specific surface area tends to be accelerated. These influences tend to be significant with impurities of alkali metals and alkaline earth metals. Further, if titanium or aluminum as an impurity are present on the surface of the silica gel or in a siloxane bond, the acidification site tends to increase, and the silica gel itself may show an unfavorable catalytic effect when used as a catalyst carrier or an adsorbent.
Kim et al. (Ultrastable Mesostructured Silica Vesicles Science, 282, 1302 (1998) describes the preparation of mesoporous molecular sieves of high thermal (1000° C.) and hydrothermal stabilities (more than 150 hours at 100° C.) by a supramolecular assembly pathway which relies on hydrogen bonding between electrically neutral gemini surfactants and silica precursors derived from tetraethylorthosilicates.
Accordingly, as a method for producing silica gel containing no impurities, a method of purifying a gel obtained by neutralizing an alkali silicate and a method of hydrolyzing a silicon alkoxide have been known, and particularly by the latter method, the silicone alkoxide can be purified by e.g. distillation, and it is thereby possible to obtain a silica gel having a high purity relatively easily. However, a silica gel obtained by a sol-gel method from a silicon alkoxide tends to have a small average pore diameter and a broad pore distribution in general. Further, substantially no report has been made with respect to an improvement in performances even if a hydrothermal treatment is applied to the silica gel.